Hmmm.. I don't think the _old db was created by the SQL migration - I'm pretty sure that SQL just upgrades existing dbs in place.
I think the _old db is actually the database for the 3.6.1 installation. Its name starts ePO_ rather that ePO4_, which implies a 361 db - and upgrading from 361 to 4 creates a new, clean db and migrates the data from the old one - but it leaves the old one behind: so that would fit the pattern you're seeing.
My instinct therefore is that this is your old 361 db, and therefore can be safely removed. The ePO4 installation will be using only the ePO4_ db, so tha's the one you need to be backing up, maintaining and so on.